The field of multiple access communications using the DS/CDMA signals is well known in the art. This technique permits multiple users (subscribers) to share the same spectral band by assigning different spreading codes to different users. Each spreading code spreads the bandwidth of the data signals by multiplication of the latter with the binary elements of a code sequence at a higher rate, called the chip-rate. Several well-known and often used spreading codes include the Gold codes, the Kasami sequences etc. in view of their excellent auto- and cross-correlation properties. In a multiuser receiver, such as the one at a base station in a wireless communication scenario, different users can be separated from each other in the code space by using appropriate signal processing techniques. In fact, one of the key problems faced by the receiver is that of removing the unwanted interference from other users. Several multiuser receiver structures have been developed which make use of the knowledge of the spreading codes of the various users. Some of the well known receiver structures include the simple matched filter receiver, the so-called “decorrelating” receiver, the minimum mean square error receiver and the minimum output energy receiver among others. Many of these solutions assume that the transmission medium is clean subject only to additive white Gassuian noise (viz., the AWGN channel). Actual wireless communication media are seldom of this nature due to rather severe multi path that results from various reflecting objects.
The main effect of the multi path nature of the communication channel is that, since the same transmitted signal is received through different, unknown paths with different delays and path strengths, the effective spreading codes associated with different users undergo unknown changes, and are thus unknown at the receiver. This makes separation of the desired signals from others rather difficult. Some methods exist which attempt to first estimate the multipath channel to ascertain its effect on the spreading codes and separate the various signals on the basis of estimated effective codes. These methods require probing of the channel via specially transmitted training sequences, which have an adverse effect on the throughput rate of the system.
In a DS-CDMA system, the data to be transmitted is modulated by a chip-code, which is unique to each user. The chip code is reasonably large, so that there are typically a large number of chips in one symbol duration. Clearly, the bandwidth of the signal is increased by the modulation. The chip is assumed to have a periodicity of one symbol duration, though this invention can be applied to cases when the periodicity is larger. Several users transmit simultaneously, so that the receiver has to make use of the unique chip code to separate the desired user's symbols. When the chip codes are orthogonal to each other, interference from other users is eliminated completely under ideal conditions (no channel distortions) as is well known. Since the number of users is typically large, the chip codes are not orthogonal to each other. In any case, channel distortions cause the effective chip code to be different, thereby destroying the orthogonality of the chip codes.
Optimum multiuser detectors [1] are difficult to implement because of their high complexity. Also, they require a lot of information—the effective chip codes code (changed by the channel) of all users.
The many parameters required by the optimal multiuser detector are difficult to reliably estimate in fast changing multipath scenario of typical CDMA links. Training sequences (known to the receiver apriori) can be used to assist the receiver in estimation of the desired chip codes. However, they consume a lot of bandwidth since these sequences need to be sent periodically when the channel is changing. Development of techniques that can estimate the channel/symbols in a blind manner (without training sequences) is thus of considerable practical interest. The proposed invention achieves just that, and among other things, does away with the need to estimate the effective codes (of even the desired user).
Although some methods of precoding at-the transmitter have been proposed in the art these all pertain to channel estimation. Also these methods typically rely on prior FIR filtering/encoding of the data, and are therefore unable to separate the different signals effectively. These methods also sacrifice data rate or system bandwidth in the process.